The major function of a telecommunications (“telco”) network device such as a switch, router or hybrid switch/router is to transfer network data or packets between various physical interfaces or ports on the network device. The ports are connected to external network attachments, for example, optical fibers, coaxial cables and twisted pair cables. Typically, a network device includes multiple forwarding cards (i.e., printed circuit boards or modules) coupled with the external network attachments through the physical interfaces. Network packets are transferred between forwarding cards through a switch fabric.
Typically, network devices transfer network data with other network devices over the external network attachments in accordance with the same clock signal (i.e., synchronously). The clock signal may be a Building Integrated Supply (BITS) Line supplied to both network devices or one network device may use a timing signal provided by the other network device over a network attachment. Regardless of which clock signal is used, a central timing subsystem with the network device is generally used to distribute timing reference signals to all cards and components involved in synchronous data transfer, for example, all port cards. It is crucial that components and cards within the network device transfer data according to the same synchronized timing signals, as transferring data at different times, even slightly different times, may lead to data corruption, the wrong data being sent and/or a network device crash. Distributing clock signals, therefore, must be done carefully to insure that the clock signal received by each component is not skewed with respect to the clock signals received by other components.
Since timing distribution is critical to network device operation, network devices often include redundant central timing subsystems. Redundancy, further complicates the distribution of timing reference signals and increases the potential for skew. In addition, if a failure of the primary central timing subsystem is detected such that a switch over to the secondary central timing subsystem is initiated, it is important that the components receiving the timing reference signals do not experience sudden phase shifts in the timing reference signals from the two central timing subsystems, as phase shifts and other noise on clock signals may also lead to data corruption or a network device crash.
Many network devices also use an independent clock signal to transfer data through the internal switch fabric, and again, a switch fabric central timing system is often used to distribute the timing reference signals to components and/or cards involved with data transfer through the switch fabric. In addition, in network devices with multiple processors, processor timing reference signals are often sent from a processor central timing system to each processor to allow the processors to synchronize their processes. Distributing multiple timing reference signals consumes considerable routing resources in the network device and again increases the likelihood of skew between clock signals.